feat: support 'this' type on methods and properties

[mattjohnsonpint]

Fixes #2904

Changes proposed in this pull request:

• Fixes support for polymorphic this type on class methods and properties
• Reports a "not implemented" for fields declared with the this type (would be nice, but is complicated)
• Adds compiler tests for this feature

To clarify, the this type was already implemented, but was returning the base type instead of the calling instance type.

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